A hydrostatic transmission is known from U.S. Pat. No. 5,062,267, in which a radially outer axial piston pump fixed to a casing is arranged coaxially with a radially inner axial piston motor provided on a rotor rotatably supported in the casing, and by guiding the piston of the axial piston pump and the piston of the axial piston motor by separate swash plates, the axial piston motor, which is connected to an output shaft, is driven by a working oil discharged by the axial piston pump, which is connected to an input shaft, thus outputting the rotation of the input shaft via the output shaft at a different speed.
In an expander using high-temperature, high-pressure steam as a working medium, when sliding parts of a piston, a cylinder, a swash plate, an output shaft, etc. housed in a casing are lubricated with oil, if the working medium leaks past the sliding parts of the piston and the cylinder into the interior of the casing, oil mist is mixed with the working medium within the casing. For example, in a Rankine cycle system in which a working medium circulates within a closed circuit formed from an evaporator, an expander, a condenser, and a supply pump, if the working medium that has been mixed with and contaminated by oil within the casing of the expander is returned to the system, it then affects the functions of the evaporator and the condenser, and there is the problem that it becomes necessary to increase the size of a filter employed as a countermeasure to separate or remove the oil from the working medium. Moreover, if the oil is mixed with the working medium, the lubrication performance is affected, and it is therefore desirable to separate the oil from the working medium immediately.